What are gravitational lenses?
Ninety years ago, Arthur Eddington undertook an expedition to West Africa to confirm Albert Einstein's general theory of relativity. Eddington was not, of course, an ethnologist or geologist but an astrophysicist and he observed the solar eclipse there on 29 May 1919. This enabled him to photograph stars in the region around the Sun - stars which would otherwise have been obscured by the Sun. In doing so, Eddington found one of the effects predicted by Einstein's theory: the stars no longer appeared to be in their true position but seemed to have shifted slightly. The light of the stars was curved by the gravitational field of the Sun, so that they were apparently in a different position when compared to observations made when the Sun was not in front of them.
The principle of a gravitational lens
Experimental confirmation of the general theory of relativity
In this way, Eddington achieved the first experimental confirmation of the general theory of relativity: a massive body like our Sun, for example, bends space. See also the Astronomy Question from week 23: 'Will Lisa find gravitational waves and prove Einstein right?' That is why light rays do not pass such a mass in a straight line but are bent in a similar way to an optical lens.
Gravitational lenses are therefore massive bodies - stars, galaxies or whole clusters of galaxies, for example - which bend the light from objects lying behind them. Depending on its form and the distribution of its mass, such a gravitational lens can produce changes in brightness, shifts in apparent position, distorted images or multiple images of the object under observation. Conversely, the analysis of such images allows us to draw conclusions about the form and mass of the gravitational lens and permits us to investigate the distribution of mass throughout the universe.